Editor—Chronic progressive external ophthalmoplegia (CPEO) is a common clinical manifestation of mitochondrial cytopathies characterised by ophthalmoplegia and ptosis.1 Approximately two-thirds of CPEO patients harbour a large, heteroplasmic, mitochondrial DNA (mtDNA) deletion.2 Some other CPEO patients carry a point mutation in the mitochondrial tRNA genes. Twelve point mutations in six mitochondrial tRNA genes have been reported to date in association with CPEO (Mitomap at http://www.gen.emory.edu/mitomap.html). Among the 12 mutations, 5703C→T in the tRNA^Asn gene has been functionally characterised.3 Here we report functional analysis of 5877G→A in tRNA^Tyr identified in a patient with CPEO4 using ρ⁰ cells that lack mtDNA.

A 45 year old woman had moderate degrees of ptosis, external ophthalmoplegia, and proximal muscle weakness from the age of 28. She had no sensorineural hearing loss, ataxia, pigmentary retinopathy, hypogonadism, or mental retardation. She had episodic diarrhoea of unknown aetiology. An ECG showed atrioventricular conduction block, while EEG, brain CT, and brain MRI showed no abnormalities. An exercise loading test of 15 watts for 15 minutes on a bicycle ergometer5 raised her serum lactate from 6.9 mg/dl to 24.0 mg/dl (normal, less than 18.0 mg/dl), and her serum pyruvate from 0.6 mg/dl to 1.4 mg/dl (normal, less than 1.3 mg/dl), thereby increasing the lactate to pyruvate ratio from 10.7 to 17.1 (normal, less than 13.8). A biopsy specimen obtained from the biceps brachii showed 4.0% ragged red fibres and 0.7% cytochrome …